1. Field of the Invention
The present invention pertains to blocked polyurethane prepolymers. These prepolymers can be used as components of compositions for forming sound- and vibration-damping coatings on thin metal substrates. Such compositions are relatively low in viscosity, are comparatively inexpensive to produce, are storage-stable, and are capable of providing cured coatings upon heating that are highly adhesive and chip-resistant.
2. Discussion of the Related Art
The use of various blocked polyurethane and blocked polyisocyanate materials as components of coating and adhesive compositions, especially compositions useful for forming vibration- and sound-damping coatings on vehicle undersides and the like, is well known in the art. The blocked polyurethane or blocked polyisocyanate becomes reactive with other components of the coating composition upon heating to provide a cured coating. Examples of patent publications describing such compositions include U.S. Pat. No. 5,155,154, U.S. Pat. No. 4,977,201, U.S. Pat. No. 5,087,664, EP 884341, EP 583377, U.S. Published Application 2003/0232908, U.S. Pat. No. 6,559,193, U.S. Pat. No. 5,741,824, Japanese Publication No. 2001-059067, and Japanese Publication No. 2003-268302. However, workers in the field believe that further improvements in such systems are still needed, particularly with respect to viscosity and storage stability. That is, it would be highly desirable to develop blocked polyurethane prepolymers that not only are sufficiently low in viscosity to enable the prepolymers and coating compositions containing such prepolymers to be applied easily to substrate surfaces by spraying but also exhibit good stability during storage (e.g., no phase separation).